1. Field of the Invention
The present invention relates to a modular knee prosthesis system, that is a system of implantable prosthesis used to replace a natural knee and which includes a series of intramedullary stems having different configurations including different lengths and different valgus angles. The valgus angle and the stem length and stem diameter can be varied by the surgeon immediately prior to the implantation of the device which allows greater surgical flexibility in the implantation of a total knee prosthesis.
2. Prior Art
Total knee prostheses have been in use for some time. These prostheses generally comprise a tibia component, a femoral component and a patellar component. The femoral component of the prosthesis generally comprises spaced-apart condylar portions and a connector joining the condylar portions. The tibial component of the prosthesis is constructed to receive the condylar portions of the femoral component, that is, the condylar portions are in direct contact and are supported by the tibial component. The femoral contact surface of the tibial component is made from a biocompatible plastic material such as ultra high molecular weight polyethylene. The patellar portion of the prosthesis is generally a ultra-high molecular weight polyethylene button, which may be metal backed or reinforced, and which is affixed to the natural patella and rides in a depression on the metal surface of the femoral component. Typical of such prosthesis is the prosthesis shown in U.S. Pat. No. 4,298,992.
The prostheses of the type shown in the above mentioned patent are generally available with or without an intramedullary stem. The intramedullary stem is used to lend lateral stability to the prosthesis and is inserted into the medullary canal of the femur. The stems are set at an angle to the vertical which duplicates the valgus angle in the human anatomy. The valgus angle is the angle between the center line of the femur and an imaginary vertical line extending from the distal femur to the center of the femoral head. This angle is generally somewhere between 5.degree. and 9.degree..
When selecting a prosthesis for implantation, the surgeon does not necessarily know the valgus angle precisely and may or may not know whether or not a prosthesis with an intramedullary stem would be desired for the particular patient. It is the general rule of thumb that the least complicated prosthesis which requires the least removal of bone from the patient would be the prosthesis of choice to be implanted.
Modular prosthesis such as those shown in U.S. Pat. Nos. 4,404,691 and 4,578,081 have been proposed. The prosthesis disclosed in these patents have stems or shanks which may be extended in length, but do not provide for a prosthesis with no stem or variation in the valgus angle of the stem.
U.S. Pat. No. 4,822,366 discloses a modular knee prosthesis in which the valgus angle of a femoral stem including a femoral stem extension can be varied to some degree. The configuration of the device requires extensive removal of bone from the distal femur to implant the device.